The invention concerns a method and device using an electrode rod for a pH meter with a measuring end for immersion into a liquid test sample, wherein the measuring end comprises a pH measuring element and electrical feed lines in the electrode rod extend towards the pH measuring element.
An electrode rod of this type is disclosed e.g. in “IQ 240 Benchtop/Portable pH Meter” by I.Q. Scientific Instruments, Inc., San Diego, Calif., USA, 2001.
When substances, e.g. proteins, are measured using modern methods of instrumental analytics, the measuring result is not only influenced by the substance itself but also by the measuring surroundings. In particular, in NMR spectroscopy of substances in aqueous solution, the pH value considerably affects the obtained spectra.
A correlation between the measuring result and the measuring surroundings is sometimes possible, i.e. the influence of the measuring surroundings can be extracted from the measuring result, thereby obtaining information about the actual substance. It is generally simpler and more accurate to examine the substance in certain, defined measuring surroundings. Towards this end, the measuring surroundings of the substance must be adjusted before the measurement.
Solutions are particularly suited as measuring surroundings, since the substance is homogeneously distributed therein and the amount of a liquid test sample (that is the substance in a solution in the measuring surroundings) can be easily adjusted.
As a first step, adjustment of the measuring surroundings often includes adding a certain amount of a standard solution and/or a buffer solution. In a further step, the pH value is in general iteratively adjusted, typically to pH 7.0.
When a sufficient amount of the substance and therefore also of the liquid test sample is available, the test sample is disposed into a large sample container, e.g. a beaker in order to adjust the pH value. An electrode rod of a pH meter and two pipettes for adding acid and base are immersed into the test sample. Acid or base is added until a desired pH value is obtained, which is simultaneously measured by the pH meter. This method is illustrated e.g. in the company leaflet “Basic Titrino 794” by Metrohm Ionenanalytik.
In many applications of instrumental analytics, only small amounts of the substance to be measured are available and hence also of the liquid test sample, since the substance may generally not be excessively diluted before measurement in order to produce exact measuring results. Typical amounts of test sample for the analysis of mouse urine using NMR spectroscopy are e.g. 0.5 to 2 ml. The liquid test sample, whose pH value is to be adjusted, is thereby provided in a small and narrow sample container.
In this case, iterative adjustment of the pH value in accordance with prior art is very difficult. An electrode rod of a pH meter is initially immersed into the narrow sample container holding the test sample, and the pH value is determined. The electrode rod is then pulled out and a small amount of acid or base is added to the test sample. The test sample is briefly mixed and the electrode rod is subsequently immersed again to check the pH value. The electrode rod is removed again and acid or base is added in the required amounts. This is continued until the desired pH value is reached.
Due to the narrow sample container (e.g. a 2 ml glass vial with an opening width of approximately 5 mm), the electrode rod (smallest available outer diameter 3.8 mm) cannot remain in the sample container during titration of acid or base, since there is not enough space for pipettes.
For this reason, the laboratory equipment (electrode rod, mixer, acid pipette, base pipette) must be removed and reinserted from above the test sample a plurality of times during preparation of a small amount of liquid test sample, which renders preparation of such a test sample very time-consuming and therefore expensive.
It is therefore the underlying purpose of the present invention to facilitate and accelerate preparation of liquid test samples, in particular adjustment of the pH value, for small amounts of test sample and in narrow sample containers. Automation of the preparation of the liquid test samples should also be simplified.